When Castle Island Brewing Co. opened its doors to the public for the first time on Dec. 11, there was a crowd of 50 people waiting outside. Castle Island has spent close to no money on advertising. The brewery’s website is a work in progress, directing to a Facebook page with a follower count in the low four figures. The fanfare was unexpected.

“To have that kind of reception waiting for us was huge,” says founder Adam Romanow. “I think a big surprise for us, in a positive sense, was just how enthusiastic people would be to come and buy the beer.”

Romanow, who used to brew at White Birch Brewing in Hooksett, N.H., says he spent five years raising the funding to get Castle Island off the ground. The 20,000-square-foot Norwood brewery represents the culmination of that work. Castle Island has six employees, four working full time and two part time.

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There’s an incongruity, though, between the brewery’s name and location. Initially, the plan was to open in South Boston, where Romanow’s relatives have founded family businesses going back three generations. Romanow lives in the neighborhood, a block from Castle Island. But Southie lacks manufacturing space, and property owners make more money turning buildings into condos. “Southie was an aspiration that was probably unrealistic,” he says.

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The building he found in Norwood housed a janitorial supply company. “It was filled with these huge racks of soap and rolls of paper towels,” says Romanow. “But it was a nice big, open space.”

But Southie still influences the beer in some ways: Candlepin — a hoppy session ale, low in alcohol but packed with flavor — is named in homage to the legendary bowling alley on Broadway. There’s also the heavier Keeper, an IPA, and a new double IPA called Causeway, brewed with Galaxy, Columbus, and Simcoe hops.

Romanow likes to put his own twist on popular styles. Castle Island’s most unusual beer is TBD, a combination of an IPA and a stout.

“It’s somewhere in between,” says Romanow. “It’s got a lot of chocolate; it’s got a lot of piney hops in there. It’s a pretty good way to warm up.”

Castle Island Brewing Co. (31 Astor Ave., Norwood, 781-951-2029) is open Thursday, Friday, and Saturday (go to facebook.com/castleislandbeer for hours). This week, the beer entered the Boston market. Look for it at Social Wines, South Boston (617-268-2974, socialwinesbos.com), and Wegman’s, Burlington (781-418-0700, wegmans.com).